Coronary artery disease caused by atherosclerosis is the leading cause of death in Europe and the US. A major risk factor in the development of atherosclerosis is hypercholesterolemia with an elevated level of cholesterol-carrying low-density-lipoprotein (LDL cholesterol or LDL-C) particles in the circulation. The metabolism of LDL cholesterol particles is highly regulated to balance the synthesis of cholesterol with the dietary intake to serve the need for cholesterol in the body. A key regulator in the turnover of LDL cholesterol is the LDL receptor (LDLR) that mediates cellular uptake of LDL particles and decreases the level of LDL cholesterol in the circulation, as illustrated by patients with familial hypercholesterolemia caused by LDLR deficiency or functional impairment.
A successful strategy to reduce LDL cholesterol plasma levels is to increase the cellular levels of LDLR, and today the most widely used medication to lower LDL cholesterol is statins. Statins inhibit the synthesis of cholesterol and up-regulate the expression of LDLR, thus overall resulting in decreased amounts of circulating LDL cholesterol. However, a considerable number of patients do not respond to or tolerate statins due to various side effects. Furthermore, statins also increase the expression of proprotein convertase subtilisin-like/kexin type 9 (PCSK9) that was recently identified as a potent negative regulator of LDLR (Seidah et al., 2014), thereby counteracting the beneficial effect on the LDL cholesterol.
Targeting of PCSK9 is a recent strategy for lowering plasma LDL-C (Lagace et al., 2006). The cellular level of the LDLR is reduced due to the ability of PCSK9 to bind the LDLR thereby impairing recycling and enhancing lysosomal degradation of the receptor. PCSK9 gain-of-function mutations lead to a significant increase in circulating LDL-C due to increased degradation of LDLR. In contrast, individuals with PCSK9 loss-of-function mutations show reduced levels of LDL-C and exhibit fewer incidents of coronary heart disease. Several leading pharmaceutical companies have obtained approval for therapeutic strategies targeting PCSK9 for the alleviation of hypercholesterolemia. Administration of PCSK9-specific antibodies directed against the LDLR-binding site of PCSK9 is reported to decrease LDL-C plasma levels in Phase III clinical trials (Mullard (2015) Nat Rev Drug Discov; Sheridan (2015) Nature Biotechnology). However, the PCSK9:LDLR binding constant is in the range of 120-620 nM (Cunningham et al., 2007; Fisher et al., 2007) while the PCSK9 plasma concentration is around 6 nM (Lakoski et al., 2009), rendering it highly unlikely that PCSK9 binds LDLR directly at physiologically relevant concentrations. In addition, PCSK9 only targets LDLR in the liver and not in e.g. steroid hormone producing tissues, which also express high levels of LDLR, suggesting the requirement of a liver-specific co-receptor (Seidah et al., 2014). Thus, LDLR is most likely not the primary PCSK9 receptor. Instead, an unknown receptor (receptor X) may capture circulating PCSK9 and subsequently deliver it to LDLR. Inhibition of PCSK9 binding to this receptor is therefore a superior strategy compared to inhibition of the PCSK9:LDLR interaction.